FIG. 1 shows an example of a conventional DC-DC converter of that type. In FIG. 1, a sinusoidal wave voltage from an AC power source 1 is rectified and smoothed by a full-wave rectifying circuit 2 and a capacitor C1, to produce a DC voltage. The produced DC voltage is passed through a switching element Q1 and is applied to a primary winding P of a transformer T. The switching element Q1 is ON/OFF-controlled according to a drive signal from a control circuit 4. Although not shown, the control circuit 4 can control various circuits such as an output voltage feedback circuit, an output current feedback circuit, an output overvoltage protection circuit, an output voltage decrease detection circuit, and an overheat protection circuit.
The DC voltage generated by the capacitor C1 turns on a constant-current start-up circuit 5 consisting of a switching element Q2 made of a MOSFET, a resistor R1, a resistor R2, a Zener diode ZD1, and a diode D1. Namely, the DC voltage is passed through the resistor R1 and is applied to a gate of the switching element Q2 to turn on the switching element Q2, to thereby pass a constant current through a route of the switching element Q2, resistor R2, diode D1, and capacitor C2. This results in charging the capacitor C2. When the voltage of the capacitor C2 reaches a start-up voltage (for example, 16 V) of the control circuit 4, the control circuit 4 starts to output a drive signal to the switching element Q1.
In response to the drive signal, the switching element Q1 starts to turn on and off. When the switching element Q1 is ON, the voltage is applied to the primary winding P of the transformer T, which accumulates energy.
When the switching element Q1 is OFF, the energy accumulated in the transformer T is discharged as electrical energy from a secondary winding S of the transformer T. This voltage is rectified and smoothed by a diode D51 and a capacitor C51, to provide a required DC voltage. The transformer T has a tertiary winding C serving as a power source for the control circuit 4. A voltage generated by the tertiary winding C is rectified and smoothed by a diode D2 and the capacitor C2, to provide a source voltage for the control circuit 4.
The start-up circuit 5 consumes large power because it receives a current from the high-voltage power source. If a large start-up current is needed to shorten a start-up time of the DC-DC converter, it will involve increased energy. Accordingly, a start-up control circuit 6 having a diode 3, a resistor R3, a resistor R4, a capacitor C3, and a switching element Q3 detects a start of the DC-DC converter (corresponding to a start of the control circuit 4) according to a voltage generated by the tertiary winding C of the transformer T, and according to the detected voltage, turns on the switching element Q3. This brings a gate bias voltage of the switching element Q2 nearly to a ground voltage, to turn off the switching element Q2. This results in turning off the start-up circuit 5. In this way, the start-up circuit 5 is turned off after a start of the DC-DC converter, to reduce unnecessary power consumption.